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Those two facts present him with a strategic challenge: How does he draw sharp contrasts with Mr. Obama in a way that doesn't undermine his efforts to repair an image tarnished by months of Democratic attacks and his own missteps?

"The problem Romney has is that he has to do a number of things, some of which conflict with each other," said John Weaver, former adviser to Sen. John McCain, the last Republican presidential nominee. "He's got to be aggressive…He also has to appear empathetic," Mr. Weaver said, adding: "Those things are not easy to achieve at the same time."

The challenge for Mr. Romney, other Republicans say, will be balancing charm with attacks as he seeks to dislodge some of Mr. Obama's softer supporters and present himself as a compassionate politician.

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Kenneth Duberstein, a former White House chief of staff under Ronald Reagan, suggested the higher priority is for Mr. Romney to present himself more positively.

"We like to like our presidents," Mr. Duberstein said. "He needs to be more apple pie and less castor oil."

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Mitt Romney speaks during a rally at Valley Forge Military Academy and College on Sept. 28 in Wayne, Pa.
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Mr. Romney hinted at his approach during a radio interview that aired Friday. Asked by Washington, D.C., station WMAL whether he would bring a "sense of the jugular" to the debate, Mr. Romney said: "Well, I frequently point out the differences" between himself and Mr. Obama. "And you will hear that very pointedly expressed."

The debates are the final major landmark on the campaign calendar before Election Day. Given the large size of the expected TV audience, they could mark the last chance for Mr. Romney to reclaim his fate after watching the president open up small but continuing leads in recent polls, despite mixed economic data and pessimism among many voters about the U.S. economy.

"It's a big deal—everyone in the campaign thinks it's a big deal," said Tom Rath, an adviser to Mr. Romney and longtime Republican strategist in New Hampshire.

Mr. Duberstein said, "The first debate is the last chance to make a first impression."

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Obama campaign ad makers aren't letting Mitt Romney forget his '47%' comments while the GOP nominee himself is appearing in a new commercial making a plea to the nation's middle class.

Messrs. Romney and Obama are two of the most experienced debaters to find themselves in the presidential spotlight, though debating isn't a particular strong suit for either.

Mr. Obama faces far different expectations heading into the debates than his Republican rival. With leads in most recent polls, the president's chief aim will be to avoid a race-shifting gaffe.

The president is known both for soaring oratory and for lengthy answers that can sound professorial. He survived 19 primary-election debates and three presidential debates in 2008 without a major gaffe—and without a memorable attack on an opponent. "He is not a sound-bite debater. He tends to speak in paragraphs," said William Newnam, a debate expert at Emory University in Atlanta. The president's own low moment came during the 2008 primary, when he told his top rival, then-Sen. Hillary Clinton, "You're likable enough, Hillary" in a frosty tone that fed perceptions that he was condescending and arrogant.

Rep. Peter Roskam, an Illinois Republican who debated Mr. Obama during their shared tenure in the state legislature, said the president still exhibits "thin skin" when opponents object to his policies. He said Mr. Obama has a higher bar to meet in the three debates because he has to recapture some of the excitement from his initial bid. "He's got to knock it out of the park three debates in a row," he said. "He's basically got to put the magic back."

Mr. Romney, the veteran of 35 primary-election debates over two campaign cycles, managed to dispatch his then-top rival, former House Speaker Newt Gingrich, in part because of his strong performance in a Florida debate. But in other debates, Mr. Romney occasionally quibbled over rules and once rebuffed an attack by Texas Gov. Rick Perry by offering him a $10,000 bet—a line that fed perceptions that Mr. Romney's wealth left him out of touch with ordinary Americans.

Though the Republican primary debates generated relatively large audiences, those ratings are expected to be dwarfed by the number of people who tune in for the general-election contests. A little more than 7.6 million people watched the most highly rated debate during the primary season, according to Nielsen. In 2008, 52.4 million viewers tuned in for the first presidential debate between Messrs. Obama and McCain.

In the run-up to the first debate Wednesday, both camps have engaged in traditional pre-event posturing to damp expectations. Obama aides have reminded reporters of the president's lackluster reviews during his first White House run, and the Romney campaign has pointed out that of the two contenders, Mr. Obama is the only one who has been in a general-election presidential debate.

White House spokesman Jay Carney, who covered the last election for Time magazine, told reporters aboard Air Force One this week that the president won the Democratic nomination in 2008 "in some ways in spite of his debate performance," while Mr. Romney earned his party's nod this year "because of them."

Eric Fehrnstrom, a senior adviser to Mr. Romney, joked about the Obama campaign's efforts to raise the bar for the GOP candidate, saying that Mr. Romney "is a veritable Cicero, if the Obama camp is to be believed."

Mr. Fehrnstrom said the one-on-one format will also mark a departure for a candidate who is more accustomed to the primary format in which the candidates all crowd on stage and fight for airtime.

"During the primary debate, whenever Gov. Romney had something critical to say about President Obama, his opponents agreed with him," the Romney adviser said. "Now, he will face an opponent with a very different point of view."

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